Nitride coated silicon nitride cutting tools

Abstract

Cutting tools and cutting tool inserts having improved mechanical and chemical wear resistance under demanding conditions of machining speed, temperature, or workpiece hardness comprise a silicon nitride substrate body having at least one hard, adherent coating layer of a refractory metal nitride. The silicon nitride substrate body consists essentially of a first phase of silicon nitride and a refractory second phase comprising silicon nitride and an effective amount of a densification aid selected from the group consisting of silicon dioxide, aluminum oxide, magnesium oxide, yttrium oxide, hafnium oxide, zirconium oxide, the lanthanide rare earth oxides, and mixtures thereof.

What is claimed is: 
     
       1. A coated ceramic cutting tool comprising a densified silicon nitride substrate body having at least one adherent coating layer; said substrate body consisting essentially of a first phase of silicon nitride and a refractory second phase comprising silicon nitride and an effective amount of a densification aid selected from the group consisting of silicon dioxide, aluminum oxide, magnesium oxide, yttrium oxide, hafnium oxide, zirconium oxide, the lanthanide rare earth oxides, and mixtures thereof; said adherent coating layer consisting essentially of a refractory metal nitrides. 
     
     
       2. A coated ceramic cutting tool in accordance with claim 1 wherein said refractory metal nitride coating is selected from the group consisting of titanium nitride, vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride, tantalum nitride, and tungsten nitride. 
     
     
       3. A coated ceramic cutting tool in accordance with claim 2 wherein said refractory metal nitride coating is titanium nitride or hafnium nitride. 
     
     
       4. A coated ceramic cutting tool in accordance with claim 1 wherein said coating layer is of a thickness between about 0.1 microns and 10 microns. 
     
     
       5. A coated ceramic cutting tool in accordance with claim 4 wherein said coating layer is of a thickness between about 1.0 and about 10 microns. 
     
     
       6. A coated ceramic cutting tool in accordance with claim 1 wherein said densification aid is present in an amount of between about 1 and about 25 weight percent of said substrate body. 
     
     
       7. A coated ceramic cutting tool in accordance with claim 6 wherein said densification aid is present in an amount of between about 1 and about 5 weight percent of said substrate body. 
     
     
       8. A coated ceramic cutting tool in accordance with claim 7 wherein said densification aid is silicon dioxide. 
     
     
       9. A coated ceramic cutting tool in accordance with claim 1 wherein said densification aid is magnesium oxide. 
     
     
       10. A coated ceramic cutting tool in accordance with claim 1 wherein said densification aid is aluminum oxide. 
     
     
       11. A coated ceramic cutting tool in accordance with claim 1 wherein said densification aid is yttrium oxide. 
     
     
       12. A coated ceramic cutting tool in accordance with claim 1 wherein said densification aid is hafnium oxide. 
     
     
       13. A coated ceramic cutting tool in accordance with claim 1 wherein said densification aid is zirconium oxide.